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HAND-BOOK OF PHYSIOLOGY. 



has been just described (a, Fig. 150), divides and subdivides; its walls 

 at the same time becoming thinner and thinner, until at length they are 

 formed only of a thin membrane of areolar and elastic tissue, lined by a 

 layer of squamous epithelium, not provided with cilia. At the same 

 time, they are altered in shape; each of the minute terminal branches 



FIG. 150. Ciliary epithelium of the human trachea, a, Layer of longitudinally arranged elastic 

 fibres; 6, basement membrane; c, deepest cells, circular in form; d, intermediate elongated cells; e y 

 outermost layer of cells fully developed and bearing cilia. X 350. (Kolliker.) 



i 



widening out funnel-wise, and its walls being pouched out irregularly 

 into small saccular dilatations, called air-cells (Fig. 151, Z>). Such a 

 funnel-shaped terminal branch of the bronchial tube, with its group of 

 pouches or air-cells, has been called an infundibulum (Figs. 151, 152), 



FIG. 151. FIG. 152. 



FIG. 151. Terminal branch of a bronchial tube, with its infundibula and air-cells, from tte mar- 

 gin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; 6 6, infundibula 

 and air-cells, X 10. (F. E. Shulze.) 



FIG. 152. Two small infundibula or groups of air-cells, a a, with air-cells, 6 6, and the ultimate 

 bronchial tubes, c c, with which the air-cells communicate. From a new-born child. (Kolliker.) 



and the irregular oblong space in its centre, with which the air-cells com- 

 municate, an intercellular passage. 



The air-cells, or air- vesicles, may be placed singly, like recesses from, 

 the intercellular passage, but more often they are arranged in groups or 



